The present invention relates to a tool for coldforming operations.
Cemented carbide products are used in tools for different coldforming operations of materials like steels, copper alloys, composite materials, etc. Examples of such tools are wire drawing dies, which are a cemented carbide nib shrink fit into a steel holder. Such tools should have a hard and wear resistant surface zone which also should have the following additional properties: good thermal conductivity; low coefficient of friction, i.e., it may be self lubricating or assist lubrication with a coolant; good corrosion resistance; resistance to microcracking; and high toughness.
When using cemented carbides in tools for the forming of, e.g., copper or its alloys, chemical reactions may occur between the binder metal of the hard metal and the copper rich alloy. In order to minimize the effects of chemical wear of the cobalt binder phase and to improve the wear resistance, a cobalt (binder) content of about 3% and a grain size of &lt;1 .mu.m is used in hard metals for such applications. Often, a low carbon content close to eta phase formation is chosen. In order to maintain the fine grain size, grain growth inhibitors such as VC, Cr.sub.3 C.sub.2, etc., are used.
In order to further increase the wear resistance, the surface of the tool exposed to wear is often boronized. The boronizing treatment is itself known to the skilled artisan and is generally done by applying a paste of organic or inorganic material containing a boron compound such as boron metal, BN, B.sub.4 C, etc., on the wear surfaces of the tool and heat treating the coated tool in an argon atmosphere at 800.degree.-1100.degree. C. During this treatment, a thin gradient zone is induced into the surface zone of the hard metal tool. This zone is depleted with cobalt and also contains a boron rich phase which forms during the treatment. This makes the surface zone harder, tougher and more resistant to thermal cracking. As a result, this treatment offers an improved combination of hardness and toughness and thus increased wear resistance. This effect can also be reapplied as the surface layer is eroded. The worn surface layer of the tool is then repolished. A boron-containing paste is applied and heat treated. A tool can typically be retreated several times before it loses its internal bore geometry and the tool becomes unusable. This is the life determining factor of such a tool.